The invention relates to memory arrangements for data processing apparatus.
Data processing speed is heavily dependent on the time required to obtain information that is stored in the memory for control and processing purposes. The cost of the memory is dependent on both the required speed of operation and the storage capacity, so that it is most expensive to provide a large capacity store with a very high speed of access. One solution to this problem is to employ a hierarchical memory system for which the stores at different levels have different speeds with the largest stores having the slowest speeds, the higher speed stores being arranged to hold information which is currently in use, or is likely to be required for use in the near future. In such a system, whenever an item of information in a higher speed store is updated, it may be arranged that a corresponding item of information in a lower speed store is also updated, so as to maintain identical copies of the item in both stores. This permits the higher speed store to be overwritten at any time, to replace an item of information in it which is no longer in current use with a new item, without having first to write back the former item into the lower speed store. However, this has the disadvantage that, every time an item of information is updated, an access must be made to the lower speed store, thus tending to reduce the speed of operation of the apparatus.